Many heavy duty vehicles, such as trucks, tractors, off-road equipment, and the like, utilize a vertical exhaust system, in which the exhaust conduit extends vertically alongside the cab of the vehicle. Recently government regulations designed to reduce emissions have required that catalytic converters be incorporated in the exhaust system of certain types of truck engines. If the catalytic converter is mounted in the vertical exhaust system, there is a possibility that water, such as rain, snow or truck wash, can enter the upper end of the exhaust system and flow downwardly into contact with the catalytic converter unit. The water entering the system can be absorbed in the vermiculite matt that is located between the outer surface of the catalytic converter and the outer body of the exhaust conduit, and if the water absorbed in the matt freezes, retention can be lost between the catalytic converter and the outer body. Further, freezing of water in the catalytic converter could cause structural damage to the monolithic catalyst. As an additional problem, water flowing through the catalytic converter may tend to wash particulate material downwardly where the particulate material collects and clogs the lower surface of the catalytic converter causing premature failure of the catalytic converter.
It has been proposed to enclose the upper end of a vertical exhaust system with a rain cap to prevent rain and snow from entering the exhaust system. However, a rain cap adds to the overall cost of the exhaust system and may rattle or vibrate during engine operation. Moreover, many vehicle manufacturers will not specify rain caps because they detract from the aesthetic appearance of the exhaust system.